Laser heat treatment

ABSTRACT

A method of heat treating a surface with a laser, successive passes of the laser over the surface having a large overlap with each individual pass applying insufficient energy to obtain the desired effect on the surface but the overlapping passes applying sufficient energy. Various patterns of laser movement may be used.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/846,584, filed Jul. 15, 2013, which is herein incorporated byreference in its entirety.

TECHNICAL FIELD

Laser heat treatment.

BACKGROUND

Lasers are commonly used to heat treat the surfaces of metal objects.Typically, it is not desired to melt the surface. A laser is used tobring the surface to a temperature below the melting point of the metalbut sufficient to cause hardening when the surface is cooled. Coolingoccurs quickly when laser energy is no longer applied due to conductioninto the bulk of the material. The laser energy is applied by a laserwhich follows a path over the material guided by a computer.Conventionally, the laser supplies sufficient energy to adequately treatthe surface at least at the center of the path in a single pass. Thepath zigzags to cover the whole surface to be heat treated. The surfaceat the center of the beam is heated more than the edges, causing anuneven effect of the treatment over the surface. It is known to overlapthe path to some extent but the overlap is generally small, as too greatan overlap would cause overtreatment of the overlapped area. The paper“Predictive Modeling of Multi-Track Laser Hardening of AISI 4140 Steel”discloses simulations of overlaps of up to 50% but teaches away from 50%overlap as it finds 5/12 overlap to be preferable. When a percentage orfraction is used to indicate a degree of overlap, the percentage orfraction is used to indicate the width of the overlap of the beam insuccessive passes as a percentage or fraction of the width of the laserbeam in a single pass.

Lasers are also used in treatment applications which melt the surface.The paper “Three body abrasive wear of X12CrNiMo martensitic stainlesssteel laser alloyed with TiC” discloses 75% overlap when melting thesurface of steel to alloy the steel with TiC grains. The surface ismelted even with a single pass. This degree of overlap is notconventionally applied when not intending to melt the surface.

SUMMARY

There is disclosed a method of heat treating a surface with a laser toobtain a change in the structure of the surface without melting thesurface by directing a laser beam onto the surface, the laser beamilluminating at a point in time a portion of the surface, and moving thelaser beam to illuminate at successive points in time successiveportions of the surface, the portion of the surface and the successiveportions of the surface forming a path, the path overlapping itself topass over each point in an area of the surface to be treated more thanonce, the laser beam supplying insufficient energy to obtain the changein structure of the surface in a single pass.

In various embodiments, there may be included any one or more of thefollowing features: the width of the overlap of the path betweensuccessive passes may be greater than 50% of the width of the path in asingle pass, at least 75% of the width of the path in a single pass, orgreater than 75% of the width of the path in a single pass.

There is also disclosed a method of heat treating a surface with a laserto obtain a change in the structure of the surface without melting thesurface by directing a laser beam onto the surface, the laser beamilluminating at a point in time a portion of the surface, and moving thelaser beam to illuminate at successive points in time successiveportions of the surface, the portion of the surface and the successiveportions of the surface forming a path, the path overlapping itself topass over each point in an area of the surface to be treated more thanonce, the width of the overlap of the path between successive passesbeing greater than 50% of the width of the path.

In various embodiments, there may be included any one or more of thefollowing features: the width of the overlap between successive passesmay be at least 75% of the width of the path, or greater than 75% of thewidth of the path. The surface to be treated may be steel. The change inthe structure of the surface may be the formation of a martensitic grainstructure. The path may follow a zig-zag route, a route with stackedline passes, a looping route, or a route with S-pattern motion.

These and other aspects of the device and method are set out in theclaims, which are incorporated here by reference.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments will now be described with reference to the figures, inwhich like reference characters denote like elements, by way of example,and in which:

FIG. 1 shows an example of successive passes of a laser over a surfaceto be treated, the laser moving in opposite directions in the successivepasses;

FIG. 2 shows an example of successive passes of a laser over a surfaceto be treated, the laser moving in the same direction in the successivepasses;

FIG. 3 shows an example of successive passes of a laser over a surfaceto be treated in a prior art method, the laser moving in oppositedirections in the successive passes;

FIG. 4 shows an example of successive passes of a laser over a surfaceto be treated in a prior art method, the laser moving in the samedirection in the successive passes;

FIG. 5A shows an area of a surface treated according to an embodiment ofthe present invention;

FIG. 5B shows a cross section of the surface of FIG. 5A;

FIG. 6A shows an area of a surface treated according to a prior art heattreating method;

FIG. 6B shows a cross section of the surface of FIG. 6A;

FIG. 7A shows a surface exposed to a single pass of a laser according toan embodiment of the present invention;

FIG. 7B shows a cross section of the surface of FIG. 7A;

FIG. 8A shows a surface exposed to a single pass of a laser according toa prior art heat treating method;

FIG. 8B shows a cross section of the surface of FIG. 8A;

FIG. 9 shows the area of surface illuminated by a laser at multiplepoints of time as collectively forming a path of the laser;

FIG. 10 shows an overlap between successive passes of a laser showingwhat is meant by a percentage of overlap;

FIG. 11 shows an embodiment in which the surface is divided intosegments, each segment having zig-zag passes with 100% overlap;

FIG. 12 shows an embodiment in which the surface is divided intosegments, each segment having stacked line passes with 100% overlap;

FIG. 13 shows an embodiment with looping motion; and

FIG. 14 shows an embodiment with looping motion with passes inneighbouring strips looping in opposite directions;

FIG. 15 shows the embodiment of FIG. 14 in a context where the loopingmotion allows the heat treatment to avoid obstacles in the surface;

FIG. 16 shows an embodiment with S-pattern motion; and

FIG. 17 shows an embodiment with zig-zag motion or stacked line passesin strips, with some overlap between strips.

DETAILED DESCRIPTION

In standard laser heat treating a laser is used with a small amount ofoverlap 12 between successive passes 14, as shown in FIGS. 3 and 4. If alarge amount of overlap is used without decreasing the power orincreasing the travel speed the surface of the metal melts. The meltingof the surface damages the products and as laser heat treatment is doneafter all of the machining is completed, this is an issue. Our processgets around this issue by allowing the energy to soak into the part. Wedo this by using multiple lower-energy passes 14 with large overlaps 12,as shown in FIGS. 1 and 2. The key to the process is that eachindividual pass does not provide enough energy to fully heat treat thesteel, but it is the combination of the overlapping passes which causesthe heat treatment. If we were to run our process in a straight linewithout the overlap we would cause very little hardening in the metal;this can be seen in FIG. 7B. FIG. 7A shows a single pass 14 over asurface 10 to be treated in the disclosed method. The hardened region 16is very small if it exists at all. This differs from conventional heattreatment where if the beam was ran in a straight line over the steel itwould still provide full hardening in the area which the laser run over;this can be seen in FIG. 8B. FIG. 8A shows a single pass 14 over asurface 10 to be treated in the prior art. The hardened region 16 hasthe intended depth for a full heat treatment with the single pass.

Our process can work with various laser motion controls, be it zig-zag,stacked line passes, looping motions, or S-pattern motion. Eachconsecutive pass provides a pre-heat for the next pass. FIG. 1 shows thedisclosed method using a laser motion control in which the laser travelsin opposite directions in successive passes, and FIG. 2 shows thedisclosed method using a laser motion control in which the laser travelsin the same direction in successive passes. FIG. 3 shows a prior artmethod using a laser motion control in which the laser travels inopposite directions in successive passes, and FIG. 4 shows a prior artmethod using a laser motion control in which the laser travels in thesame direction in successive passes.

The process disclosed uses a higher travel speed or lower laser energyoutput (or a combination of both) than is used in standard laser heattreatment. By increasing the speed or lowering the laser power output weare enabling ourselves to use a greater overlap than is possible inconventional laser heat treatment. By using a greater overlap we areable to insure that the areas we heat treat do not have areas of shallowhardening. FIG. 5A shows a surface 10 treated using this method. FIG. 5Bshows a cross section of the surface of FIG. 5A showing what thehardened region 16 looks like with our process. There are areas ofshallow hardening 18 only at the ends. FIG. 6A shows a surface treatedusing a prior art method. FIG. 6B shows a cross section of the surfaceof FIG. 6A showing a hardened region 16 with shallow hardening 18 at theoverlaps between passes as well as at the ends. When FIG. 5B is comparedto FIG. 6B it becomes evident that our process is superior as theseareas of shallow hardening can be detrimental to certain products whichrequire full hardening.

Referring to FIG. 9, at a point in time a laser illuminates an area 20on the surface. At further points in time the laser illuminates portionsof the surface 20 a, 20 b and 20 c. The portions illuminated by thelaser at successive points of time define a path 22. The path 22 has awidth indicated by line 24. FIG. 4 shows the laser illuminating acircular portion of the surface but it may illuminate different shapes,for example, a rectangular portion of the surface. The laser may becontinuous or pulsed.

Referring to FIG. 10, the path of the laser returns to a portion of thesurface that it has previously illuminated, each time it does so beingreferred to as a pass over that portion of the surface. The path has awidth as indicated by line 24 on each of the passes. The passes have anoverlap indicated by line 26. The degree of overlap is represented as apercentage, where the percentage refers to the ratio of the width of theoverlap to the width of the path expressed as a percentage. In FIG. 10as shown, the width of the overlap is 75% of the width of the path sothe percentage of overlap is 75%. In an embodiment, the width of thepath may be different on one pass than on another pass. In this case,the percentage of overlap may be considered to be the ratio of theoverlap to the larger of the widths of the path. In an embodiment, apass and a successive pass may be passes of respective different pathsformed by corresponding different lasers.

The process can be used for heat treatment of metal products in anyindustry, including but not limited to, for example, oilfield equipmentor automotive parts.

As the heat from each laser pass dissipates over time, it is preferredthat each pass over a point in the surface occur within a relativelyshort time frame. For a sufficiently large surface, there may not beenough time for the laser to traverse the full width of the surfacebefore the heat from a pass dissipates, and so the width of the surfacecan be divided into strips 30, the path of the laser traversing thewidth of a strip in each pass and each strip being treated in turn.FIGS. 11-15 and 17 show various embodiments of this principle. In FIGS.11 and 12 the strips are further divided into segments 32, each segmentbeing treated with two passes of the laser with 100% overlap. In FIG. 11the overlapping passes have motion of the laser in opposite directions(zig-zag motion) and in FIG. 12 the overlapping passes have motion ofthe laser in the same direction (stacked line passes). It should benoted that the zig-zag motion and stacked line passes can also be usedwith less than 100% overlap and without dividing the strip into segments32, for example, with successive passes proceeding from one end of thestrip to the other each one having less than 100% overlap with thepreceding pass, as shown in FIG. 17. In FIG. 13 an embodiment is shownin which the laser has a looping motion giving a curved shape to eachpass 14. In FIG. 14, the passes 14 in the second strip curve in anopposite direction to the passes in the first strip. FIG. 15 shows anexample context in which the embodiment of FIG. 14 might be used,showing the curved passes 14 avoiding poles 34; curved passes could alsobe used to avoid any other gap in the area of the surface to be treated.FIG. 16 shows passes 14 having an S-pattern motion, shown here in acontext where the S-pattern motion allows avoiding two poles within asingle strip. The S-pattern motion can be used with multiple strips (notshown) as with the other motions. In FIGS. 15-17 numerals 36 beside thepasses indicate an example time order in which the passes might be made.FIG. 17 shows an embodiment with zig-zag motion or stacked line passes,with an overlap 38 between strips 30. There can also be an overlapbetween strips with other motions, including the motions of FIGS. 11 and12 where there could be an overlap between segments as well as betweenstrips.

Immaterial modifications may be made to the embodiments described herewithout departing from what is covered by the claims.

In the claims, the word “comprising” is used in its inclusive sense anddoes not exclude other elements being present. The indefinite articles“a” and “an” before a claim feature do not exclude more than one of thefeature being present. Each one of the individual features describedhere may be used in one or more embodiments and is not, by virtue onlyof being described here, to be construed as essential to all embodimentsas defined by the claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A method of heat treating a surface with one or more lasers to obtain a change in the structure of the surface without melting the surface, the method comprising: directing a beam of a laser of the one or more lasers onto the surface, the beam illuminating at a point in time a portion of the surface, and illuminating at successive points in time further portions of the surface, the portion of the surface and the further portions of the surface defining a path, the path overlapping itself or the path of another laser of the one or more lasers to pass over each point in an area of the surface to be treated more than once, each laser of the one or more lasers supplying insufficient energy to obtain the change in structure of the surface in a single pass.
 2. The method of claim 1 in which the width of the overlap between successive passes is greater than 50% of the width of each of the successive passes.
 3. The method of claim 1 in which the width of the overlap between successive passes is at least 75% of the width of each of the successive passes.
 4. The method of claim 1 in which the width of the overlap between successive passes is greater than 75% of the width of each of the successive passes.
 5. A method of heat treating a surface with one or more lasers to obtain a change in the structure of the surface without melting the surface, the method comprising: directing a beam of a laser of the one or more lasers onto the surface, the beam illuminating at a point in time a portion of the surface, and illuminating at successive points in time further portions of the surface, the portion of the surface and the further portions of the surface defining a path, the path overlapping itself or the path of another laser of the one or more lasers to pass over each point in an area of the surface to be treated more than once, the width of the overlap of the path between successive passes being greater than 50% of the width of the path.
 6. The method of claim 5 in which the width of the overlap between successive passes is at least 75% of the width of each of the successive passes.
 7. The method of claim 5 in which the width of the overlap between successive passes is greater than 75% of the width of each of the successive passes.
 8. The method of claim 1 in which the surface to be treated is steel.
 9. The method of claim 8 in which the change in the structure of the surface is the formation of a martensitic grain structure.
 10. The method of claim 1 in which the surface is divided into strips, each pass of the laser traversing the width of a respective strip, and successive passes traversing a strip overlapping within the strip.
 11. The method of claim 10 in which neighbouring strips overlap each other.
 12. The method of claim 1 in which the path follows a zig-zag route.
 13. The method of claim 1 in which the path follows a route with stacked line passes.
 14. The method of claim 1 in which the path follows a looping route.
 15. The method of claim 1 in which the path follows a route with S-pattern motion.
 16. The method of claim 1 in which the surface is divided into segments, each segment having at least two passes with substantially 100% overlap.
 17. The method of claim 16 in which neighbouring segments overlap each other. 